Last time I ran into that argument, Fidel Castro was the example given. My reply was this:
If you heard a proposal to kill Fidel Castro, would you approve? Maybe. (Though even that's not quite as simple as it sounds, when you consider things like precedent and ethical prohibitions.)
If the proposal involved dropping a hydrogen bomb on Havana, would you still approve? Of course not!
This, I claimed, sufficiently refutes the idea that getting rid of a handful of bad apples justifies the death of everyone.
It's the only absolutely reliable way of getting rid of bad leaders.
This might not be a good enough reason to oppose longevity tech, but I don't think it's easily disposed of.